Practice portal › Ray Optics and Optical Instruments › Refraction through a Prism
Asked in CBSE AIPMT 1995 · Dispersion and scattering
Idea: a lens's power depends on (μ-1), and μ differs from colour to colour, so one lens alone focuses violet and red at different points.
That colour error can only be removed if a second element carries an error of the opposite sign.
A converging lens and a diverging lens have powers of opposite sign, so their colour errors can be made to cancel while a net power survives.
Choosing the dispersive powers so that (ω₁)/(f₁)+(ω₂)/(f₂)=0 removes the spread.
So an achromatic combination is 1 convex and 1 concave lens.
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